It has long been known that a relationship exists between the measured initial gate current and threshold shift of a transistor particularly for transistors under high stress conditions.
The papers, "Characterization of Electronic Gate Current in IGFET's Operating in the Linear and Saturation Regions", P. E. Cottrell et al, 1977 Device Research Conference, Ithaca, New York, June 1977, "Hot-Electron Emission in N-Channel IGFET's", P. E. Cottrell et al, IEEE Journal of Solid-State Circuits, April 1979, and "Hot-Electron Design Considerations for High Density RAM Chips", R. R. Troutman et al, IEEE Transactions on Electron Devices, Vol. ED-27, pp. 1629-1639, August 1980, teach that a correlation exists between the measured initial gate current and long term device threshold degradation.
In "IGFET Hot Electron Emission Model", A. Phillips, Jr., et al, 1975 IEDM Digest, paper 3.3, pages 39-42 apparently teaches that if gate current could be measured, as opposed to being calculated via a model, threshold voltage shifts could be determined. This paper indicated what the expected threshold shift would be over a matter of days based on calculated emission current for a stressed device.
An article, "Optically Induced Injection of Hot Electrons in SIO.sub.2 ", Ning and Yu, Journal of Applied Physics, December 1974, pages 5373-5378, teaches that, under radiant energy stimulation of free carriers, hot electrons injected into the gate dielectric can be measured by monitoring gate current. The article also implies that, if trapping efficiency for a particular structure is known, threshold voltage shift might be determined by gate current as a function of time.
"Threshold Instability in IGFET's Due to Emission of Leakage Electrons from Silicon Substrate Into Silicon Dioxide", by Ning, Osburn and Yu, Applied Physics Letters, Aug. 1, 1976, pages 198-200, teaches that if the number of hot electrons injected is small compared with the dielectric trap density then the threshold shift rate is proportional to the emission current density. Here, as in the Phillips et al article, actual gate current was too small to be measured directly.
"Substrate Current-A Device and Process Monitor", Abbas, 1974 IEDM, paper 17.7, pages 404-407. "Detection of Hot-Electron Injection and Trapping in FET Devices", El-Kareh et al, IBM Technical Disclosure Bulletin, January 1976, pages, 2455-2456. "Hot Electron Monitor" Auriemma et al IBM Technical Disclosure Bulletin, October 1976, pages 1632-1633. "MOSFET Hot-Electron Effect Characterization", Kriese et al, IBM Technical Disclosure Bulletin, November 1976, pages 2119-2120 also teaches general monitoring techniques for studing hot-electron effects.
The above prior art, at best, only teaches that a relationship exists between the measured initial gate current and threshold shift of a transistor particularly for transistors under high stress.
The present invention is clearly distinguishable from this prior art, for it teaches that the rate of change of gate current in a transistor under normal operating conditions can be measured and that this rate of change can be used to determine, over a very short period of time, the long term threshold shift.